Electricity meter reading technology has developed in recent years to include, in addition to traditional manual reading of a meter dial, a capability for remote reading. In a customary remote reading arrangement, optical sensing of the meter rotor is performed. A disk on the rotor shaft, which rotates at a rate proportional to power usage, conventionally may contain an aperture near its periphery, the disk acting as a shutter between a light emitting diode and a photosensor. The aperture occupies a set portion of the total disk area within the travel path of the optical sensor.
During travel of the aperture through the path of the optical sensing area, illumination of the photosensor occurs, whereby a voltage threshold in the sensing circuit, set to correspond to the light exposure condition, is exceeded. The non-aperture portion of the disk blocks such light transmission and the sensing circuit does not exceed the voltage threshold during non-exposure. Transition between the dark condition and light saturation of the detector is relatively abrupt and finite.
For each revolution, the sensing circuit should provide at least two transitions between signal logic levels, in correspondence to passage through the light path, in succession, of a non-apertured portion of the disk, the apertured portion, and again a non-apertured portion. The detector circuit counts the transitions and correlates the count with the number of shaft revolutions to obtain a measure of power usage. Signals representing such measurement are communicated by way of telephone lines or data lines to a remote power company location.
In an alternative arrangement, the rotor disk is provided with a darkened light absorbing area instead of an aperture. Light is emitted onto the rotor disk and is either reflected from the non-darkened area of the disk to the detector or absorbed by the darkened area. Power usage is determined similarly by correlating a count of transitions sensed by the detector circuit. A meter reading scheme using such a variation is disclosed in U.S. Pat. No. 4,327,362, issued to Hoss on Apr. 27, 1982. Amplification of the detected signal is compared with a voltage reference, the comparator output being buffered and applied to a counter. Hoss includes a hysteresis circuit for providing a bilevel comparator reference voltage to overcome errors which might be created at the boundaries of the darkened area. Such errors may result from attenuation of light in the course of transmission, a possibility with the use of a low power source, and presence of spurious signals produced by noise and varying ambient temperatures which change component characteristics.
Detection by conventional optical sensor electricity meter systems during periods of low power usage, when the disk is moving slowly or is stationary, is subject to inaccuracy due to false registration errors that are not as likely to occur at other times. Disk transition zones between light and dark, include regions wherein the detector operates in a linear manner; that is, between light saturation and total absence of light. Linear detector operation transpires when an optical shutter partially blocks the sensor at the borders of the darkened light absorbing areas. At such times the conducting current in the sensing phototransistor is proportional to the amount of light to which the phototransistor is exposed. Mechanical vibration or electrical noise occurring when the disk is positioned at such operating zones can cause the detector falsely to sense alternate light and dark conditions. When the rotor is relatively stationary, a tendency for the rotor to creep backwards even a small amount may register false disk movement.
Such problems have been addressed in the past by providing a plurality of sensors offset in the disk path of travel. Two sensors are positioned to have outputs logically interconnected to register the disk aperture shutter passing both of the sensors but not to register the shutter alternating within one of the sensors. Such an arrangement would ensure that registration of a transition occurs only if the disk traverses a predetermined minimum distance of travel. An example of a multiple sensing arrangement is disclosed in U.S. Pat. No. 4,827,123, issued to Gray on May 2, 1989.
The provision of multiple sensors introduces cost and design disadvantages. There must be at least two emitters, two detectors or two of each. Additional electronics is required to interface with an existing single optical sensor system.
Another prior art approach is to apply the detector output of a single sensor system to a comparator having positive feedback or a Schmitt trigger having predefined threshold hysteresis. The Hoss patent, previously identified, exemplifies use of hysteresis in the comparator circuit.
Hysteresis in a logic gate or comparator is the ability to have a low threshold for a falling input signal and high threshold for a rising input signal. In order to function in this manner, the gate must have a memory of the previous state of the input signal. In the Hoss circuit, for example, electrical hysteresis is provided by supplying a comparator with positive feedback from an output to a non-inverting input through a feedback resistor to form a latch or gate circuit. The input signal to the comparator is thus reinforced by the output signal. Thus, the comparator "remembers" the last input state until the input signal swings far enough in the opposite direction to overcome the feedback signal supplied at the input of the comparator. Such operation requires the optical emitter to be energized nearly continuously and several stages of amplification for this purpose. Otherwise, the voltage level applied to the input of the comparator would swing through the dark threshold when the emitter is turned off resulting in an erroneous input being applied to the comparator and causing the comparator to forget its previous state. The power consumption necessary for such emitter energization is, at a minimum, a disadvantage and in some cases intolerable.
A further disadvantage of the prior art systems is the small transition zone between the light saturation and dark regions. The hysteresis introduced by the Hoss circuit, intended to correct for errors that result from attenuation of light in the course of transmission, will be insufficient to cope with many instances of mechanical vibration and reverse creep of the disk as the disk sensor is in the linear operation region of the detector for a very small percentage of a disk revolution. In a narrow zone of linear operation, there is a relatively large change in the amount of light to which the sensor is exposed for a small increment of shaft rotation. Such change can exceed the amount for which the hysteresis voltage threshold can compensate.